[Test Proxy] Improve variables API documentation

[mccoyp]

Description

Prompted by an issue the Storage team ran into, when kwargs.pop("variables") fails in live test pipelines (because the test proxy isn't running, and variables therefore aren't provided).

In addition to recommending a default value for kwargs.pop, this simplifies the use of variables by using the setdefault method to query the dictionary, or update it, in the same line -- without any need to check for emptiness or live-running status of the test!

@scbedd

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.
  • Verified using the Storage test that prompted this change